The present invention relates to a device for determining characteristics of a biomass, that is to say of all media composed of biological cells. It also relates to a method used in this device.
A real time estimation of the characteristics of the biomass is essential for the optimum control of fermentation processes used in the food production, pharmaceutical and chemical industries and in biotechnologies. For a description of the general principle of the real-time estimation of a biomass, it will be possible to refer usefully to the article xe2x80x9cDielectric permittivity of microbial suspensions at radio frequencies: a novel method for the real-time estimation of microbial biomassxe2x80x9d by C. M. Harris et al., published in the review xe2x80x9cEnzyme Microb.technol., Volume 9, March 1987.
The document EP0281602 divulges an equipment for determining a biomass, comprising mutually spaced electrodes provided for being placed in the medium in electrical contact with the latter; and means for generating a signal dependent on the capacity between the electrodes, at a predetermined frequency or in a predetermined frequency range between 0.1 MHz and 10 MHz.
These electrodes comprise a first pair of electrodes for injecting current into the medium, surrounding a second pair of current electrodes, and are disposed in a probe including amplification means and connected to an electronic conditioner.
This conditioner comprises means for applying an alternative voltage, at the predetermined frequency, across the current electrodes, means for supplying a current signal indicative of the instantaneous current in the current electrode circuit, means for providing a voltage signal indicative of the instantaneous voltage at the terminals of the voltage electrodes; and means for determining the ratio between the value of the voltage signal and the value of a component in quadrature of the current signal, or vice-versa, in order to provide a signal dependent on the capacity.
However, this signal is necessarily also dependent on the frequency of the alternative voltage applied to the voltage electrodes, which implies maintaining this frequency constant during a measuring sequence.
The purpose of the invention is to propose a device for determining the electrical characteristics of a biomass, which makes it possible to obtain directly a signal representative of the capacity independently from the excitation frequency of the electrodes. Furthermore, another purpose of the invention is to obtain a device for determining electrical characteristics which is provided with a passive measuring probe having no electronic amplification means.
Furthermore, there is a growing need for measuring devices making it possible to determine the state of freshness and preservation of food products with a basis of biological cells, such as slices of fish or ham, which are offered for consumption.
Said objectives are achieved with a device for determining characteristics of a biomass, comprising:
a probe provided for being applied to a medium containing biological cells, said probe comprising means for injecting a current into said medium,
means for reading the voltage applied to said medium, and means for measuring the injected current,
a conditioner comprising means for providing a galvanically isolated alternative voltage to be applied to said current injection means, and means for processing signals respectively representative of the current injected into said medium and of the voltage read by the voltage reading means, in such a way as to deliver measurement signals respectively of the capacity and of the conductance of said medium.
According to the invention, the processing means comprise:
a measuring bridge using the null method designed to process a signal representing the injected current and a signal representing the read voltage applied respectively to a reference branch and to two opposing branches, and
means for automatically controlling this bridge on the basis of the conductance measurement signal.
Unlike the impedance measuring devices of the prior art using a measuring bridge using the null method in which the impedance to be measured is actually inserted, in the measuring device according to the invention, the measuring bridge using the null method is disposed downstream of circuits delivering signals respectively representing the injected current and the voltage at the terminals of the impedance to be measured. In this way isolation problems are solved because this arrangement allows a floating bridge circuit and a preliminary amplification of the measurement signals delivered by the probe.
In a particular embodiment, the measuring bridge comprises:
a reference branch including a reference resistor to which is applied the signal representing the injected current,
a first opposing branch including an adjustable opposing resistor and a second opposing branch including an adjustable opposing capacitor, the signal representing the read voltage being applied on these opposing branches, and
amplification means having their input connected to said reference and opposing branches and delivering a null measurement signal.
In a device according to the invention further comprising means for delivering a signal representing the voltage read by the voltage reading means and means of delivering a signal representing the current injected by the current injection means, the conditioner further comprises a first modulator inserted between the output of the means of delivering the signal representing voltage and the first opposing branch, this first modulator being controlled by the conductance measurement signal in such a way that the null measurement signal is substantially zero.
In a particular embodiment, the probe comprises four wires connecting the current injection means and the voltage reading means to four terminals of connection means for connecting with the conditioner, and two additional wires respectively connecting the terminals of a current measuring resistor disposed inside said probe to two other terminals of said connection means.
In a first embodiment, the current injection means comprise two current electrodes for injecting current into the medium and the voltage reading means comprise two voltage electrodes for reading the voltage applied to the medium.
The current measuring resistor is then inserted between one of the current injection electrodes and one of the wires of the probe is connected via the connection means to a floating earth of the conditioner. The probe furthermore preferably comprises a compensating resistor inserted between one wire of the probe and the other current injection electrode.
In a second embodiment, the current injection means and the voltage reading means are produced in the form of a pair of measuring electrodes comprising a first measuring electrode connected to both a first wire and a second wire of the probe and a second measuring electrode connected to both a third wire and a fourth wire of the probe.
This simplified embodiment can be envisaged when the measuring ranges and the sought accuracies permit it. The current measuring resistor can be inserted between the second measuring electrode and one of the wires of the probe connected via the connecting means to a floating earth of the conditioner, and the probe can further comprises a compensating resistor inserted between the first measuring electrode and a wire of the probe.
In both of said embodiments, the current measuring resistor is preferably disposed in the vicinity of the electrodes of the probe.
This particular arrangement of the measuring probe has as an advantage the fact that this probe can be entirely passive and not include an amplifier, unlike the probe described in the document EP0281602 which includes amplification electronics. It then becomes possible to design probes having a very small diameter, for example having a diameter of 12 mm.
Furthermore, the conditioner of the device according to the invention can be easily checked by replacing the measuring probe with a passive standard probe consisting of a resistor and a capacitor.
Several probe geometries can he envisaged within the context of the invention, both for the embodiment having four electrodes and for the embodiment having two electrodes. It is thus possible to provide a probe in which the electrodes are disposed on a flat support at the end of a cylindrical body of the probe and disposed substantially parallel with each other.
The electrodes can also consist of concentric annular elements, or they can even be disposed on a tubular body or on a substantially flat body.
The automatic control means can be designed to control the bridge on the basis of the capacity measurement signal. The conditioner then further comprises a second modulator inserted between the output of the means of delivering the signal representing voltage and the opposing capacitor, said second modulator being controlled by the capacity measuring signal in such a way that the null measuring signal is substantially zero.
In a practical embodiment of the device according to the invention, the processing means further comprise, at the output of the measuring bridge, a first channel and a second channel respectively, each one comprising synchronous detection means and first integrators delivering the capacity and conductance measurement signals respectively, these synchronous detection means being controlled by the output signal of oscillator means.
The probe comprises only passive components and is connected in a detachable manner to the conditioner. The conditioner further comprises a first differential amplifier and a second differential amplifier electrically connected to the probe and provided for delivering the current signal and the voltage signal respectively.
It should be noted that it is also possible to provide, within the context of the present invention, active probes including one or more active components of the conditioner.
According to another aspect of the invention, a method is proposed for determining characteristics of a biomass, used in the device according to the invention, comprising:
an injection of an alternative current, at a predetermined frequency into a medium containing biological cells, by current injection means,
a measurement of the current injected into said medium,
a measurement of the voltage at the terminals of voltage reading means disposed in the vicinity of the current injection electrodes, and
a processing of the signals representing the current injected into said medium and the read voltage respectively, in such a way as to deliver measurement signals of the capacity and of the conductance respectively of said medium.
This method is characterised in that the processing of the current and voltage signals includes a null method using a measuring bridge comprising, on the one hand, a reference branch on which the signal representing the current is applied and, on the other hand, two opposing branches on which the signal representing the voltage is applied, these opposing branches respectively comprising an adjustable resistive component and an adjustable capacitive component, and this measuring bridge being automated to deliver a measurement signal of capacity and a measurement signal of the conductance of the medium.
Thus, in the present invention, the measurement of the resistance and of the capacity of the medium is determined by a null method, on the basis of the action that it is necessary to carry out in order to cancel out the real part and the imaginary part of the representation of the current passing through the biomass. With this measuring method, it is not necessary to control the amplitude of the voltage at the terminals of the transmitting electrodes, unlike the measuring method described in the document EP0281602 for which it is imperative to maintain the amplitude on the receiving electrodes constant.
For a description of the general principle of impendence measuring by the null method with the use of four electrodes, it will be possible to refer usefully to the article xe2x80x9cFour-Electrode Null Techniques for Impedance Measurement with High Resolutionxe2x80x9d by H. P. SCHWAN and C. D. FERRIS in the publication xe2x80x9cThe Review of Scientific Instrumentsxe2x80x9d, Volume 39, No 4, April 1968.
The use of a measurement by the null method procures numerous advantages among which is the fact that the measurement of capacity is direct and does not depend on frequency. Furthermore, the measurement of capacity is not very sensitive to harmonics and procures a very good resolution.
The opposition is achieved by processing a voltage signal taken from the terminals of voltage measuring electrodes, via a reference resistor and an opposing capacitor. This opposition is carried out just at the output of differential measuring amplifiers respectively supplying a voltage signal and a current signal, and the opposition amplitude is automatically servo-controlled by means of modulators.